The irradiation method of a particle beam therapy system is roughly divided into a broad irradiation method where beams are concurrently irradiated onto the whole diseased site of a patient as an irradiation subject and a scanning irradiation method where a diseased site is scanned with a beam. The scanning irradiation method includes a spot-scanning method, a raster-scanning method, and the like; herein, these methods will collectively be referred to as a scanning irradiation method. In order to realize the scanning irradiation method, apparatuses and control methods suitable for its irradiation method are required. The front end portion from which a charged particle beam is actually irradiated also needs to be contrived in order to realize the scanning irradiation method. The front end portion from which a charged particle beam is irradiated is referred to as an irradiation system, an irradiation field forming apparatus, an irradiation head, an irradiation nozzle, or the like; however, herein, the front end portion will be referred to as an irradiation apparatus.
In the scanning irradiation apparatus utilized in a conventional particle beam therapy system, in order to raise the accuracy of an irradiation position when the diseased site of a patient is irradiated, there has been proposed a configuration where a vacuum region or a region of gas such as helium, which is lighter than air, is ensured so that scattering of a beam is suppressed and hence the beam size is reduced (e.g., refer to Patent Document 1). The portion where a vacuum region or a gas region is ensured is referred to as a chamber (such as a beam transport chamber or a gas chamber) or a duct (such as a vacuum duct); however, it can be understood that the chamber and the duct are substantially the same. Accordingly, the foregoing portion will be referred to as a duct herein. A portion, in the duct, through which a charged particle beam passes is referred to as a window. The window is referred to as an isolation window (isolation membrane) or a beam outlet window depending on a patent document; however, herein, the most downstream window in the orbit of a charged particle beam will be referred to as a beam outlet window.
The scanning irradiation apparatus in a conventional particle beam therapy system will be explained with reference to FIG. 7. The scanning irradiation apparatus is configured with a vacuum duct 1 for ensuring a vacuum region; a window (beam outlet window) 7, in the duct 1, through which a charged particle beam passes; beam scanning apparatuses 5a and 5b for performing scanning with a charged particle beam; beam position monitors 3a and 9 for measuring the position of a charged particle beam; and a dose monitor 8 for measuring a beam dose.
Next, the operation of the scanning irradiation apparatus in a conventional particle beam therapy system will be explained. A charged particle beam accelerated by an accelerator passes through a beam transport apparatus and then is introduced into the vacuum duct 1 (in the case of FIG. 7, from the upper part to the lower part of the drawing). The charged particle beam passes through the window 7 of the vacuum duct 1 and then exits into the air; the first beam position monitor 3a confirms the beam irradiation position. At a further downstream position, the irradiation direction of the charged particle beam is controlled by the beam scanning apparatuses 5a and 5b formed of a scanning electromagnet or the like. The charged particle beam is irradiated in such a way as to follow the center line (dashed line) drawn in FIG. 7 and adjusted in such a way as to ultimately head for an isocenter (irradiation reference point) 11 when the beam scanning apparatuses 5a and 5b do not perform any control.
Usually, one of the beam scanning apparatuses 5a and 5b performs scanning in the X axis direction, and the other performs scanning in the Y axis direction. The dose of the charged particle beam is measured by the dose monitor 8 disposed at a further downstream position; the position of the beam is confirmed again by the second beam position monitor 9 disposed at a still further downstream position. The charged particle beam is finally irradiated onto the disease site of a patient as an irradiation subject. As illustrated in FIG. 7, some of the dose monitors 8 and the second beam position monitors 9 can be moved in the beam axis direction by a driving device 21 in accordance with the position and the size of the diseased site of a patient 10b as an irradiation subject. Additionally, as illustrated in FIG. 8, it is conceivable that, in order to ensure as much vacuum region as possible in a region where a charged particle beam passes, vacuum ducts 4a and 6a are additionally provided in the irradiation apparatus in a conventional particle beam therapy system.
It is conceivable that a conventional scanning irradiation apparatus is utilized and as illustrated in FIG. 8, a vacuum region or a region of gas such as helium that is lighter than air is ensured so that there is reduced beam scattering caused by air within that region. However, there has been a problem that because beam scattering caused by air is merely one of conditions that determine the size of a beam, the scanning irradiation apparatus illustrated in FIG. 8 cannot realize a small beam size that is required to perform a practical scanning irradiation. Hereinafter, further explanation will be made.
FIG. 9 is a schematic diagram for explaining the relationship between the scattering angle and the beam spot diameter. When hitting on some obstacle, a beam advancing straight is scattered and propagates with some spread. The foregoing spread is referred to as a scattering angle and expressed by θ (radian) in FIG. 9. The beam spot diameter at a position that is r (distance) apart from the obstacle is approximately rθ, as illustrated in FIG. 9. In a conventional scanning irradiation apparatus, the window 7 and the first beam position monitor 3a that are disposed at a more upstream position than the scanning electromagnet correspond to the obstacles. In other words, a charged particle beam scatters at the position of the window 7 and propagates with a spread thereafter.
In the conventional technology illustrated in FIG. 7 or FIG. 8, there has been a problem that because the obstacle that causes the scattering of a beam is located far from the isocenter as an irradiation point, i.e., because the distance r in the schematic diagram in FIG. 9 is long, the beam spot diameter becomes large and hence there is not obtained a beam size small enough to be applied to practical scanning irradiation. As illustrated in FIG. 8, it is conceivable that, in order to ensure as much vacuum region as possible in a region where a charged particle beam passes, vacuum ducts 4a and 6a are additionally provided in the irradiation apparatus in a conventional particle beam therapy system. The foregoing configuration can certainly reduce beam scattering caused by air. However, this configuration does not make the position of the window 2a where a beam firstly scatters closer to the irradiation subject; therefore, it does not solve the problem completely. Additionally, the number of windows through which a charged particle beam passes becomes three (the isolation windows 2a and 2b and the beam outlet window 7), i.e., the number of windows increases, which has been a cause that makes the beam size large.
Another problem posed in the case where the vacuum region is merely added as illustrated in FIG. 8 will be explained with reference to FIG. 8. The size of the place where the diseased site of a patient as an irradiation subject is located is not always the same. For example, with regard to the cross section of the place where the diseased site of a patient as an irradiation subject is located, it should be considered that, as illustrated in FIG. 8, the size of the cross section differs depending on the patient, for example a patient 10a (type 1) or a patient 10b (type 2). Compared with the case of the patient 10a (type 1), in the case of the patient 10b (type 2), the air gap, i.e., the distance of the air path through which a charged particle beam passes becomes long.
Originally, this air gap is unnecessary; as explained with reference to the schematic diagram in FIG. 9, the distance r between the obstacle and the irradiation point should be as short as possible. There has been a problem that, even though the distance between the irradiation point and an obstacle, such as the dose monitor 8 or the second beam position monitor 9, which causes the scattering of a beam should be as short as possible, the air gap cannot be shorten in the configuration illustrated in FIG. 8 due to the system's functional restrictions.
Patent Document 1: Japanese Patent Application Laid-Open No. 2007-268035
Patent Document 2: Japanese Patent Application Laid-Open No. 2007-229025